Devices used to trace and detect audio signals on metallic conductors are commonly referred to as tone detectors, inductive amplifiers, or tone probes. These devices are used by telecommunications personnel, Local Area Network (LAN) personnel, Wide Area Network (WAN) personnel, cable television personnel, or by any persons wishing to locate and identify a particular conductor. Tone probes couple to a conductor electromagnetically, amplify the signal, and output the audio signal to a speaker or headset so the user can hear the signal on the conductor.
Tone probes are often used in conjunction with a tone generator or tone sender. The tone generator transmits a known or characteristic electrical signal at one or several audio frequencies. The tone generator is attached to a conductor at a known location and transmits over the conductor to a location where the conductor is to be identified using the tone probe. The distance between the transmitter and identification location may be from several feet to many miles. If the distance from the tone generator is many miles, the locating signal will be attenuated. Additionally, the conductor that is to be identified may be in a cable with many other metal conductors or wires. Often other wires and conductors are carrying other signals that induce noise onto the pair that is carrying the locating signal. Thus, the undesired noise signals may be at a higher level than the locating signal and the locating signal will be undetectable by a user using most common tone probes.
Conventional tone probes may be either unfiltered or filtered. An unfiltered tone probe converts all electrical signals that it receives in an audio frequency range to an audio signal. A tone signal sent by the tone generator can be easily detected if the environment in which the signal is being received is noiseless. The user must discriminate between noise and signal if noise is present where the conductor is being identified. The signal may not be identified at all if noise signals are so strong that the user cannot discriminate the tone.
Filtered probes assist the user by attempting to reduce the unwanted noise signals, thereby making the signal of interest more distinctive and easier to discriminate and locate. Filtration available today consists of low pass, high pass, band pass, and comb filtration.
Low pass and high pass filtration filters electrical signals above and below a certain frequency, as implied by the name. Tone probes designed with these types of filters have a cutoff frequency that is selected to reduce most of the noise, while passing the signal of interest. The filter also passes audio frequencies below or above the cutoff. These filters are often inadequate because noise can still dominate in those portions of the audio spectrum.
Some probes use band pass filtration that passes frequencies only related to the signal of interest and attenuates all other signals to a certain percentage. Band pass filters attenuate the noise signals better, but are often insufficient if the signal of interest is low in amplitude and the noise signals are high. Noise often still dominates the output of the tone probe in this case.
Much of the predominant noise on cables installed by various utilities is caused by power influence, or 60 Hz AC induction. Noise in these situations exists at 60 Hz and its harmonics. That is, energy in the electromagnetic spectrum will exist at multiples of 60 Hz (e.g., at 60, 120, 180, 240, 360, and continuing up the frequency spectrum at multiples of 60 Hz). Comb filters in tone probes are designed to notch out or attenuate the 60 Hz harmonics and pass all other frequencies of interest. If other noise is present, or the power influence is not precisely at 60 Hz, this method may become inadequate as well.
Accordingly, a continuing search has been directed to the development of methods and apparatuses that can filter out noise while amplifying a desired signal, so that tones may be readily detected in relatively noisy environments.